Use Taulman t-glase and after a light sanding with really fine paper (optional really, but go for it if you can), spray it with polyurethane varnish or something similar. Check out the article here.
I am aware of several "clear" filaments for a ABS or PLA printer. They, however, have a cloudy or frosted glass appearance. I do not believe this is possible to eliminate but I believe it can be reduced.
Are there effective ways to make a print have a more transparent appearance?